This invention relates to linear actuators and, more particularly, to an improved linear actuator of the type disclosed in copending U.S. application Ser. No. 579,432 filed on May 21, 1975 by Messrs. Halfhill and Brunner as a divisional of copending U.S. application Ser. No. 486,408 filed on July 8, 1974, now U.S. Pat. No. 3,922,718.
The unique linear actuator disclosed in copending application Ser. Nos. 486,408 and 579,432 makes use of the principle that a roller frictionally engaged with the cylindrical surface of a drive shaft will be rotated about its axis by rotation of the drive shaft when such axis is parallel to the axis of the drive shaft, and will additionally be moved linearly in a direction parallel to the axis of the drive shaft when the roller axis is oblique to the axis of the drive shaft.
In general terms, therefore, the linear actuator disclosed in the aforesaid copending application Ser. Nos. 486,408 and 579,432 includes a drive shaft having a cylindrical surface, means for rotatably mounting the drive shaft to a support frame for rotation of the drive shaft about its axis, means for rotating the drive shaft about its axis, a carriage to be driven, a roller, means for mounting the roller to the carriage with the roller being rotatable about a first axis and pivotable about a second axis perpendicular to the first axis, means for mounting the carriage to the support frame with the carriage being movable relative to the support frame along the predefined linear path and with the roller being in frictional engagement with the cylindrical surface of the drive shaft whereby the roller is caused to rotate about its first axis by rotation of the drive shaft when the first axis is parallel to the axis of the drive shaft and is additionally caused to move along the predefined linear path during rotation of the drive shaft when the first axis is oblique to the axis of the drive shaft, and means for controllably pivoting the roller about its second axis to control movement of the roller and thus the carriage along the predefined linear path.
As disclosed in the aforesaid copending application Ser. Nos. 486,408 and 579,432, the linear actuator may be included in and form part of a magnetic disk drive. More specifically, disk drives generally include a drive spindle for rotating one or more magnetic recording disks. A head carriage is associated, with each disk and may include two electromagnetic heads, one for each surface of the disk. Since information is recorded on the disk in concentric tracks which are spaced very closely adjacent one another, it is necessary to provide a linear actuator for the head carriage that is capable of moving the heads thereon to and from selected tracks on the disk at high speed and with great precision. Energization of the linear actuator to cause movement of the head-carriage assembly in the appropriate direction and speed is controlled by a suitable servo control system.
In view of the precision and speed required in positioning the head-carriage assembly of a disk drive, it is desirable that the roller be pivotable about its second axis, as above defined, with a minimal amount of torque. The amount of torque required is directly related to the frictional resistance to pivoting the roller as engaged with the cylindrical surface of the drive shaft. Such frictional resistance to pivoting is, in turn, a function of the force holding the roller in frictional engagement with the cylindrical surface, the materials of the roller and cylindrical surface, and the contact area between the roller and cylindrical surface. By "contact area" it is meant that portion of the peripheral surface of the roller and the cylindrical surface of the drive shaft which are in mutual engagement at any instant of time.
The roller disclosed in copending application Ser. Nos. 486,408 and 579,432 has a peripheral surface of lineal cross-sectional configuration with the cross-section being taken along a plane including the axis of the roller therein. Although this configuration is acceptable, the contact area is relatively large thereby requiring a relatively large amount of torque to pivot the roller about its second axis. On the other hand, since the size of the contact area is inversely proportional to the stress on the roller and drive shaft, a roller with a peripheral surface of lineal cross-sectional configuration, as above-defined, will result in a relatively low level of stress.
In a disk drive, if the torque required to pivot the roller is relatively large, the power required to supply such torque would detract from the power savings otherwise normally attributed to this type of linear actuator, as discussed in the aforementioned copending applications. Additionally, the need for a substantial amount of torque might slow down the actuator thereby slowing the positioning capabilities thereof and otherwise rendering the actuator less accurate.
It would be desirable, therefore, if the roller's peripheral surface were optimized in shape in the sense that the torque required to pivot the roller about its second axis is made as small as possible consistent with an acceptable level of stress on the roller and cylindrical surface of the drive shaft for the particular materials thereof and normal force engaging the roller against such cylindrical surface.